1. Field of the Invention
The present invention relates to magnetic powder, a manufacturing method of magnetic powder and bonded magnets. More particularly, the present invention relates to magnetic powder, a manufacturing method of the magnetic powder and a bonded magnet which is manufactured, for example, using the magnetic powder.
2. Description of the Prior Art
For reduction in size of motors, it is desirable that a magnet has a high magnetic flux density (with the actual permeance) when it is used in the motor. Factors for determining the magnetic flux density of a bonded magnet include magnetization of the magnetic powder and the content of the magnetic powder to be contained in the bonded magnet. Accordingly, when the magnetization of the magnetic powder itself is not sufficiently high, a desired magnetic flux density cannot be obtained unless the content of the magnetic powder in the bonded magnet is raised to an extremely high level.
At present, most of practically used high performance rare-earth bonded magnets are the isotropic bonded magnets which are made using the MQP—B powder manufactured by MQI Inc. as the rare-earth magnetic powder thereof. The isotropic bonded magnets are superior to the anisotropic bonded magnets in the following respect; namely, in the manufacture of the bonded magnet, the manufacturing process can be simplified because no magnetic field orientation is required, and as a result, the rise in the manufacturing cost can be restrained. On the other hand, however, the conventional isotropic bonded magnets represented by those manufactured using the MQP—B powder involve the following problems.
The conventional isotropic bonded magnets do not have a sufficiently high magnetic flux density. Namely, because the magnetic powder that has been used has poor magnetization, the content of the magnetic powder to be contained in the bonded magnet has to be increased. However, the increase in the content of the magnetic powder leads to the deterioration in the moldability of the bonded magnet, so there is a certain limit in this attempt. Moreover, even if the content of the magnetic powder is somehow managed to be increased by changing the molding conditions or the like, there still exists a limit to the obtainable magnetic flux density. For these reasons, it is not possible to reduce the size of the motor by using the conventional isotropic bonded magnets.
Although there are reports concerning nanocomposite magnets having high remanent magnetic flux densities, their coercive forces, on the contrary, are so small that the magnetic flux densities (for the permeance in the actual use) obtainable when they are practically used in motors are very low. Further, these magnets have poor heat stability due to their small coercive forces.
The conventional bonded magnets have low corrosion resistance and heat resistance. Namely, in these magnets, it is necessary to increase the content of the magnetic powder to be contained in the bonded magnet in order to compensate the low magnetic properties (magnetic performance) of the magnetic powder. This means that the density of the bonded magnet becomes extremely high. As a result, the corrosion resistance and heat resistance of the bonded magnet are deteriorated, thus resulting in low reliability.